This invention relates to planographic printing and provides a substrate for a planographic printing member and a planographic printing member per se. The invention particularly, although not exclusively, relates to lithographic printing.
Lithographic processes involve establishing image (printing) and non-image (non-printing) areas on a substrate, substantially on a common plane. When such processes are used in printing industries, non-image areas and image areas are arranged to have different affinities for printing ink. For example, non-image areas may be generally hydrophilic or oleophobic and image areas may be oleophilic. In xe2x80x9cwetxe2x80x9d lithographic printing, a dampening or fountain (water-based) liquid is, in general, applied initially to a plate prior to application of ink so that it adheres to the non-image areas and repels oil based inks therefrom. In xe2x80x9cdryxe2x80x9d printing, ink is repelled from non-image areas due to their release property.
Image and non-image areas can be created by processes that include a step of exposing a layer of image material on the surface of the substrate to radiation. The exposure to radiation creates solubility differences in the image material corresponding to image and non-image areas. During development, the more soluble areas are removed, leaving a pattern on the substrate corresponding to the image.
The properties of lithographic plates are highly dependent on the substrate itself and particularly its uppermost surface, since it is this surface which must bond with image material prior to imaging of the plate but allow release of soluble image material during development and, furthermore, it must be non-ink accepting and thereby define non-image areas of the plate.
Other important properties affected by the substrate may include the following:
a) the shape of dots (or other printing areas) on the plate;
b) the resolution obtainable using the plate;
c) the range of dots obtainable using the plate;
d) the exposure latitude of the plate;
e) the ink-water balance of the plate;
f) the number of prints obtainable using (and, therefore, the durability of) the plate;
g) the speed of the plate;
h) the tendency of the plate to pick-up ink in non-image areas;
i) the aesthetics of the plate pre- and post-development.
The accurate reproduction of dots (or other printing areas) in terms of their size and/or shape (e.g., properties a) to c)) is becoming more and more important for use in applications such as stochastic printing and/or high quality color printing. Accordingly, plates that can accurately reproduce dots and have other advantageous properties are highly desirable. In addition, there is some evidence of a movement in the printing field towards plates with a wider exposure latitude (property d)). Properties such as e), f) and h) are properties that always need to be optimized whilst property i) is a desirable property to optimize, because good plate aesthetics may affect a printer""s perception of the quality of a plate and make it easier for the printer to inspect the quality of an image produced, for example pre- or post-development.
One of the most common substrates used in lithographic printing comprises an aluminum base layer that is treated to make it ready for use. For example, the aluminum may be roughened, for example by electrograining, anodized and then conditioned by chemical means, for example by treatment with water, a solution of phosphate or silicate salt, or a polycarboxylic acid.
However, one problem associated with the use of an electrograined and anodized aluminum substrate is its poor ability to accurately reproduce dots. Another problem is the expense (both economically and environmentally) of preparing the substrate.
It is also well-known to prepare a substrate by applying a hydrophilic layer on a support of, for example, aluminum or plastic. Numerous different hydrophilic layers have been proposed that possess a whole range of chemistries and morphologies. However, very few printing plates of the type described have been commercialized. Those that have tend to have poor properties and are generally used for low quality, short run length applications. Thus, a need exists for a substrate for a printing member that can accurately reproduce dots, is economically and environmentally easy to produce, and yet can be used for high quality, long run applications.
This invention addresses the problems associated with printing members. In particular, in one embodiment the invention is a substrate for a planographic printing ember, the substrate comprising:
a support; and
a hydrophilic layer;
in which:
the hydrophilic layer comprises a particulate material and a binder for the particulate material; and
the hydrophilic layer has a surface roughness of about 0.1 xcexcm to 2 xcexcm.
In a preferred embodiment, the hydrophilic layer comprises a mixture of two particulate materials, preferably alumina and titanium dioxide. In another embodiment, the invention is a planographic printing member comprising the substrate and an image layer over the hydrophilic layer. In other embodiments, the invention is a method for forming the substrate and a method for forming the planographic printing member.
The substrate is suitable for preparing a planographic printing member having a resolution of 10 xcexcm or less and in which the dots having a roundness of less than 2 are formed.